Address Classes and Their Ranges

Before flexible masks, IPv4 used classful addressing. The first bits of an address placed it into a class — A, B, C, D, or E — and the class fixed how much of the address was network versus host.

Although modern networks use classless addressing (CIDR), the Network+ exam still expects you to recognize the classes and their ranges.

Class A covers first octets 1 to 126. It uses an 8-bit network portion and a 24-bit host portion, giving very large networks with millions of hosts each.

Its default mask is 255.0.0.0, also written /8. Class A was meant for the largest organizations and was assigned sparingly.

Class B covers first octets 128 to 191. It splits the address as 16 bits network and 16 bits host, supporting up to about 65,000 hosts per network.

Its default mask is 255.255.0.0, or /16. Class B suited medium-to-large organizations such as universities and big companies.

Class C covers first octets 192 to 223. It uses 24 bits for the network and only 8 bits for hosts, allowing 254 usable hosts per network.

Its default mask is 255.255.255.0, or /24. Class C was assigned to small networks, and these ranges are extremely common in home and office setups.

Class D spans first octets 224 to 239 and is reserved for multicast — one-to-many delivery to a group of subscribers.

Class E spans 240 to 255 and is reserved for experimental use. Neither class is assigned to ordinary host interfaces, so you will not configure devices in these ranges.

You may notice Class A stops at 126 and Class B starts at 128. The 127 range is set aside for loopback testing.

The address 127.0.0.1 always refers to the local device itself. Pinging it confirms that the local TCP/IP stack is working, independent of any cable or network.

The class is actually defined by the leading bits of the first octet, not the decimal value directly:

• Class A starts with bit pattern 0
• Class B starts with 10
• Class C starts with 110
• Class D starts with 1110

These patterns produce the decimal ranges you memorized above.

Each assignable class has a default mask that defines its natural split:

• Class A: 255.0.0.0 (/8)
• Class B: 255.255.0.0 (/16)
• Class C: 255.255.255.0 (/24)

Knowing the default mask for a given first octet helps you quickly estimate network size on the exam.

Classful addressing was wasteful. An organization needing 300 hosts had to take a whole Class B (65,000 hosts), leaving tens of thousands unused.

This inefficiency accelerated IPv4 exhaustion and motivated CIDR (Classless Inter-Domain Routing), which lets administrators size networks precisely with any mask length.

In practice, networks no longer rely on rigid classes — CIDR lets you use a /27 or /22 freely. Yet the class ranges remain useful shorthand and appear on exams and in documentation.

When you see a first octet of 10, think large Class A range; 172.16, think Class B; 192.168, think Class C. These mental shortcuts speed up daily work.

Lock in the assignable ranges: A is 1-126, B is 128-191, C is 192-223. After that, D (224-239) is multicast and E (240-255) is experimental, with 127 reserved for loopback.

A reliable way to remember the gaps is that 127 sits between A and B, breaking the otherwise continuous sequence.

Identify the class from the first octet.

You mapped the IPv4 address classes.

• Class A: 1-126, mask /8 — huge networks.
• Class B: 128-191, mask /16 — medium networks.
• Class C: 192-223, mask /24 — small networks.
• Class D: 224-239 multicast; Class E: 240-255 experimental.
• 127 is reserved for loopback (127.0.0.1).
• Classful waste led to CIDR and classless addressing.